本文選題：Nb微合金化 + NM500耐磨鋼　； 參考：《北京科技大學(xué)》2015年博士論文

【摘要】：NM500低合金高強(qiáng)度耐磨鋼適用于低、中沖擊載荷工況,服役環(huán)境要求其不但具有較高的硬度和強(qiáng)度,同時還需較高的沖擊韌性。目前國內(nèi)生產(chǎn)的該級別耐磨鋼沖擊韌性普遍較低,從而導(dǎo)致耐磨性能較差,如何在保證國產(chǎn)NM500耐磨鋼硬度、強(qiáng)度的前提下,提高其沖擊韌性,進(jìn)一步提高其使用壽命,是目前國產(chǎn)NM500的主要研發(fā)方向。針對上述問題,本論文工作在國產(chǎn)NM500化學(xué)成分的基礎(chǔ)上添加不同含量的微合金元素Nb,系統(tǒng)研究了Nb含量變化對實驗鋼的析出相轉(zhuǎn)變熱力學(xué)、相變動力學(xué)、熱處理工藝優(yōu)化、強(qiáng)韌化機(jī)制及抗沖擊磨粒磨損性能等方面的影響,獲得了具備高硬度、高強(qiáng)韌性及抗沖擊磨損性能的新型低合金高強(qiáng)度耐磨鋼化學(xué)成分及相應(yīng)的熱處理工藝。 基于Thermo-calc熱力學(xué)軟件對含Nb NM500耐磨鋼中析出相的類型、析出溫度及析出量進(jìn)行了計算,結(jié)果表明：實驗鋼中隨著Nb的含量由0.018%增加到0.078%,富含Nb的MC型碳化物的析出溫度顯著提高,由1150℃提高到1300℃,同時析出量也明顯增加,這有利于通過細(xì)晶強(qiáng)化提高實驗鋼的沖擊韌性。在低溫回火條件下,MC相、M7C3相、MC_ETA相和MC SHP相碳氮化物析出相會在淬火馬氏體基體上彌散析出,產(chǎn)生二次硬化,使實驗鋼因回火而造成的硬度下降得到一定的補(bǔ)償,為較高硬度的獲得提供一定保障。 實驗鋼不同奧氏體化溫度下的奧氏體晶粒長大規(guī)律研究表明,隨著Nb質(zhì)量分?jǐn)?shù)的增加,當(dāng)奧氏體化溫度在850～950℃溫度區(qū)間時,Nb的細(xì)化晶粒作用比較明顯,當(dāng)奧氏體化溫度在1000～1200℃溫度區(qū)間時,Nb的細(xì)化晶粒作用減弱,實驗鋼的奧氏體粗化溫度為950℃。奧氏體化時間對淬火組織硬度的研究表明,奧氏體化時間在10～20min區(qū)間時,隨奧氏體化時間的增加,淬火組織硬度增加,當(dāng)奧氏體化時間超過20min時,淬火組織硬度隨奧氏體化時間的增加而逐漸降低,為使實驗鋼熱處理后的組織硬度達(dá)到NM500的級別要求,奧氏體化時間應(yīng)低于40min。實驗鋼動態(tài)連續(xù)冷卻轉(zhuǎn)變曲線(CCT曲線)的研究結(jié)果表明,在冷卻過程中,實驗鋼主要存在先共析鐵素體、珠光體+貝氏體、馬氏體等三個轉(zhuǎn)變區(qū)域,為使實驗鋼獲得適當(dāng)?shù)鸟R氏體+貝氏體比例,在獲得較高硬度、強(qiáng)度的同時,也具有較高的沖擊韌性,冷卻速度選擇在5～8℃/s比較合適。 實驗鋼在850～890℃范圍內(nèi)保溫20～40min后水淬,并在200～250℃范圍內(nèi)進(jìn)行30～60min的低溫回火后空冷,獲得的組織為回火板條馬氏體+少量殘余奧氏體,可以使實驗鋼獲得優(yōu)良的硬度和強(qiáng)韌性配合。在此熱處理工藝條件下,4組實驗鋼均達(dá)到國外企業(yè)生產(chǎn)的該級別耐磨鋼的綜合性能:含Nb量為0.043%的2#實驗鋼經(jīng)850℃保溫30min后水淬,再經(jīng)250℃回火60min后空冷,獲得的組織為回火板條馬氏體+少量殘余奧氏體,組織布氏硬度值為484、抗拉強(qiáng)度Rm=1652MPa、屈服強(qiáng)度Rp=1412MPa、斷后延長率δ=10.8%、室溫和-40℃沖擊功值分別為53.3J和51.3J,達(dá)到了NM500低合金高強(qiáng)度耐磨鋼的標(biāo)準(zhǔn)要求,并具有優(yōu)良的沖擊韌性,超過了國外廠家生產(chǎn)的同級別耐磨板的沖擊韌性,為該淬火與低溫回火熱處理工藝下的最佳成分和熱處理方案。 實驗鋼經(jīng)等溫淬火與低溫回火后的組織為回火馬氏體+黑色針狀下貝氏體。實驗鋼在850～930℃范圍保溫20-40min后淬入溫度為330～370的50%NaNO2+50%KNO3熔鹽中保溫5-20s后水淬,之后在200℃回火30min后空冷,均可獲得優(yōu)良的硬度和強(qiáng)韌性配合,并表現(xiàn)出比淬火與低溫回火熱處理工藝更好的綜合性能。在此熱處理工藝參數(shù)條件下,含Nb質(zhì)量分?jǐn)?shù)為0.043%的實驗鋼不但綜合性能好,而且各性能指標(biāo)非常穩(wěn)定,波動較小,易于熱處理過程中的控制,因此,添加0.043%Nb為最佳選擇。 通過對實驗鋼經(jīng)熱處理后的試樣模擬挖掘機(jī)鏟斗等的沖擊磨粒磨損實驗的研究表明,在中等沖擊載荷附加磨粒磨損的條件下,塑性變形區(qū)域占磨損表面的90%左右,由塑性變形造成的塑變疲勞和剝落是造成失重的主要原因。因此,在具備較高的硬度和強(qiáng)度的前提下,具備較高的沖擊韌性,能表現(xiàn)出更好的抗沖擊磨粒磨損能力。不同組織在沖擊磨粒磨損下的磨損結(jié)果表明,在中等沖擊磨粒磨損條件下,回火板條馬氏體+黑色針狀下貝氏體比回火板條馬氏體+少量殘余奧氏體具有更加優(yōu)越的抗沖擊磨粒磨損性能。在本實驗鋼成分條件下,含Nb量為0.043%的2#實驗鋼無論是采用淬火+低溫回火還是等溫淬火+低溫回火熱處理工藝,都表現(xiàn)出最佳的硬度和強(qiáng)韌性配合及性能穩(wěn)定性,能表現(xiàn)出最佳的抗沖擊磨損性能,所以添加0.043%的Nb為最佳選擇。
[Abstract]:NM500 low alloy high strength wear resistant steel is suitable for low and medium impact load conditions. The service environment requires not only the high hardness and strength, but also the high impact toughness. At present, the impact toughness of this grade wear resistant steel is generally low, which leads to poor wear resistance, and how to ensure the hardness and strength of the domestic NM500 wear resistant steel. On the premise of degree, improving its impact toughness and further improving its service life is the main research and development direction of domestic NM500. In this paper, we add different content of microalloyed element Nb on the basis of the chemical composition of domestic NM500, and systematically study the thermodynamics and phase transition of Nb content change on the precipitation phase of experimental steel. The chemical composition and heat treatment process of a new low alloy and high strength wear resistant steel with high hardness, high strength and toughness and impact wear resistance are obtained by the dynamics, the optimization of heat treatment process, the mechanism of strengthening and toughening and the abrasion resistance of the impact abrasive.
Based on the Thermo-calc thermodynamic software, the precipitation temperature and precipitation of the precipitated phase in Nb NM500 resistant steel were calculated. The results showed that as the content of Nb increased from 0.018% to 0.078% in the experimental steel, the precipitation temperature of MC carbide rich in Nb increased significantly from 1150 to 1300, and the precipitation increased obviously. It is beneficial to improve the impact toughness of the experimental steel through fine grain strengthening. Under the condition of low temperature tempering, the precipitation of MC phase, M7C3 phase, MC_ETA phase and MC SHP phase carbonitride will disperse and precipitate on the quenched martensite matrix, producing two times of hardening, which makes the hardness decrease of the experimental steel due to tempering, and the higher hardness is obtained. For a certain guarantee.
The study of austenite grain growth at different austenitizing temperatures of experimental steel shows that, with the increase of Nb mass fraction, when the austenitizing temperature is at the temperature range of 850~950 C, the grain refining effect of Nb is more obvious. When the austenitizing temperature is at the temperature range of 1000~1200 C, the refined grain effect of Nb is weakened and the austenite of experimental steel is austenite. The somatic coarsening temperature is 950. The study of austenitizing time on the hardness of quenched tissue shows that when austenitizing time is in the range of 10 ~ 20min, the hardness of the quenched tissue increases with the increase of austenitizing time. When the austenitizing time exceeds 20min, the hardness of the quenched tissue decreases with the increase of austenitizing time, which makes the hot place of the experimental steel. The hardness of the tissue is up to the level of NM500. The austenitizing time should be lower than the dynamic continuous cooling transition curve (CCT curve) of 40min. experimental steel. The results show that in the cooling process, the experimental steel mainly existed three transition regions, such as first eutectoid ferrite, pearlite + bainite, martensite and so on, in order to make the experimental steel suitable for martensite. The ratio of bainite to bainite has higher toughness and higher impact toughness, and the cooling rate is more suitable at 5~8 /s.
The experimental steel is quenched at 850~890 C for 20 ~ 40min, and is cooled at low temperature of 30 ~ 60min at 200~250 C. The obtained microstructure is tempered lath martensite and a small amount of retained austenite. The experimental steel can obtain excellent hardness and strong toughness. In this heat treatment process, 4 groups of experimental steels are reached. The comprehensive properties of the wear-resistant steel produced by foreign enterprises: the 2# experimental steel with Nb 0.043% was quenched at 850 C for 30min and then cooled at 250 C after tempering for 60min. The microstructure obtained was tempered lath martensite and a small amount of retained austenite. The hardness value of the tissue Brinell was 484, the tensile strength was Rm=1652MPa, the yield strength Rp=1412MPa, and the post fracture extension. The long rate Delta =10.8%, the impact work value at room temperature and -40 C are 53.3J and 51.3J, respectively. It has reached the standard requirement of NM500 low alloy high strength wear resistant steel, and has excellent impact toughness. It has exceeded the impact toughness of the same grade wear-resistant plate produced by the foreign manufacturers. It is the best composition and heat treatment scheme for the quenching and low temperature tempering heat treatment process.
The microstructure of experimental steel after isothermal quenching and low temperature tempering is tempered martensite and black acicular bainite. The experimental steel is quenched in 50%NaNO2+50%KNO3 molten salt at 850~930 C for 20-40min after heat preservation and then quenched for 5-20s in the molten salt of 330~370. After tempering at 200 c for 30min, the excellent hardness and toughness can be obtained. It shows better comprehensive performance than quenching and low temperature heat treatment. Under the conditions of heat treatment, the experimental steel with Nb mass fraction of 0.043% is not only good in comprehensive performance, but also very stable in various performance indexes, less fluctuating and easy to control during heat treatment. Therefore, adding 0.043%Nb is the best choice.
The research on the impact abrasive wear test of the experimental excavator shovel after heat treatment of the experimental steel shows that the plastic deformation area accounts for about 90% of the worn surface and the plastic deformation caused by plastic deformation is the main cause of the weight loss under the condition of the medium impact load additional abrasive wear. On the premise of high hardness and strength, it has high impact toughness and can show better impact abrasive wear ability. The wear results of different tissues under impact abrasive wear show that the tempering lath martensite + Black needle like bainite is better than the tempered lath martensite and a small amount of remnant austenite under the medium impact abrasive wear conditions. Under the condition of the steel composition, the 2# experimental steel with the amount of 0.043% Nb has the best combination of hardness and toughness and the stability of strength and toughness, and it can show the best anti impact wear resistance. Yes, so adding 0.043% of Nb is the best choice.
【學(xué)位授予單位】：北京科技大學(xué)
【學(xué)位級別】：博士
【學(xué)位授予年份】：2015
【分類號】：TG142.1

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